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Subject[PATCH v4 4/5] block: loop: prepare for supporing direct IO
Date
This patches provides two approaches for enabling direct IO
from user space:

- userspace(such as losetup) can pass 'file' which is
opened/fcntl as O_DIRECT
- sysfs file is provided to run dio tests easily

Also __loop_update_dio() is introduced to check if direct I/O
can be used on current loop setting.

Signed-off-by: Ming Lei <ming.lei@canonical.com>
---
drivers/block/loop.c | 76 ++++++++++++++++++++++++++++++++++++++++++++++++++++
drivers/block/loop.h | 1 +
2 files changed, 77 insertions(+)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 555b895..0b1ee2b 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -421,6 +421,44 @@ struct switch_request {
struct completion wait;
};

+static void __loop_update_dio(struct loop_device *lo, bool dio)
+{
+ struct file *file = lo->lo_backing_file;
+ struct inode *inode = file->f_mapping->host;
+ bool use_dio;
+
+ /*
+ * loop block's logical block size is 512, now
+ * we support direct I/O only if the backing
+ * block devices' minimize I/O size is 512.
+ */
+ if (dio) {
+ if (inode->i_sb->s_bdev &&
+ bdev_io_min(inode->i_sb->s_bdev) == 512)
+ use_dio = true;
+ else
+ use_dio = false;
+ } else {
+ use_dio = false;
+ }
+
+ if (lo->use_dio == use_dio)
+ return;
+
+ /* flush dirty pages to avoid stale data from following dio */
+ if (use_dio)
+ vfs_fsync(file, 0);
+
+ blk_mq_freeze_queue(lo->lo_queue);
+ lo->use_dio = use_dio;
+ blk_mq_unfreeze_queue(lo->lo_queue);
+}
+
+static inline void loop_update_dio(struct loop_device *lo)
+{
+ __loop_update_dio(lo, io_is_direct(lo->lo_backing_file));
+}
+
/*
* Do the actual switch; called from the BIO completion routine
*/
@@ -441,6 +479,7 @@ static void do_loop_switch(struct loop_device *lo, struct switch_request *p)
mapping->host->i_bdev->bd_block_size : PAGE_SIZE;
lo->old_gfp_mask = mapping_gfp_mask(mapping);
mapping_set_gfp_mask(mapping, lo->old_gfp_mask & ~(__GFP_IO|__GFP_FS));
+ loop_update_dio(lo);
}

/*
@@ -627,6 +666,41 @@ static ssize_t loop_attr_partscan_show(struct loop_device *lo, char *buf)
return sprintf(buf, "%s\n", partscan ? "1" : "0");
}

+static ssize_t loop_attr_do_show_use_dio(struct device *dev,
+ struct device_attribute *attr, char *buf)
+{
+ struct gendisk *disk = dev_to_disk(dev);
+ struct loop_device *lo = disk->private_data;
+
+ return sprintf(buf, "%s\n", lo->use_dio ? "1" : "0");
+}
+
+ssize_t loop_attr_do_store_use_dio(struct device *dev,
+ struct device_attribute *attr, const char *buf,
+ size_t count)
+{
+ struct gendisk *disk = dev_to_disk(dev);
+ struct loop_device *lo = disk->private_data;
+ int err;
+ unsigned long v;
+
+ if (!lo->lo_backing_file)
+ return -ENODEV;
+
+ err = kstrtoul(buf, 10, &v);
+ if (err < 0)
+ return err;
+
+ __loop_update_dio(lo, !!v);
+ if (lo->use_dio != !!v)
+ return -EINVAL;
+ return count;
+}
+
+static struct device_attribute loop_attr_use_dio =
+ __ATTR(use_dio, S_IRUGO | S_IWUSR, loop_attr_do_show_use_dio,
+ loop_attr_do_store_use_dio);
+
LOOP_ATTR_RO(backing_file);
LOOP_ATTR_RO(offset);
LOOP_ATTR_RO(sizelimit);
@@ -639,6 +713,7 @@ static struct attribute *loop_attrs[] = {
&loop_attr_sizelimit.attr,
&loop_attr_autoclear.attr,
&loop_attr_partscan.attr,
+ &loop_attr_use_dio.attr,
NULL,
};

@@ -783,6 +858,7 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
if (!(lo_flags & LO_FLAGS_READ_ONLY) && file->f_op->fsync)
blk_queue_flush(lo->lo_queue, REQ_FLUSH);

+ loop_update_dio(lo);
set_capacity(lo->lo_disk, size);
bd_set_size(bdev, size << 9);
loop_sysfs_init(lo);
diff --git a/drivers/block/loop.h b/drivers/block/loop.h
index b6c7d21..d1de221 100644
--- a/drivers/block/loop.h
+++ b/drivers/block/loop.h
@@ -58,6 +58,7 @@ struct loop_device {
struct mutex lo_ctl_mutex;
struct kthread_worker worker;
struct task_struct *worker_task;
+ bool use_dio;

struct request_queue *lo_queue;
struct blk_mq_tag_set tag_set;
--
1.9.1


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